1. Field of the Invention
The present invention relates to a portable telephone unit and more particularly to a portable telephone unit being capable of preserving data updated during a conversation.
2. Description of Related Art
In a portable telephone unit, there are several tens of bytes of data that are renewed every second such as billing information and conversation time, but there are some types of such data which are held in an internal memory. Further, as such a memory, an electrically erasable and writable read-only memory (ROM), which is a so called an electrically erasable programmable read-only memory (EEPROM), is used in general.
Now, in a portable telephone unit, it is made so that a charging type battery for a power source can be removed comparatively easily for the purpose of exchange or the like. Further, the battery gets out of place sometimes by a shock and so on.
Now, in case a memory of parallel interface is used as an EEPROM for holding said data, when a battery is removed, such a fact is detected by a voltage detection circuit and said data is written in the EEPROM immediately. Further, the power source for the period when the write is executed is secured by the charging voltage of a capacitor.
In a portable telephone unit, however, LSI chips are used widely for the purpose of miniaturization of an apparatus and so on. Therefore, current power consumption during the period when data is written in the EEPROM is increased, thus resulting in that the capacity of a capacitor for supplying power during the write period has to be made larger, which prevents miniaturization of a telephone set.
Further, when a memory of series interface is used as the EEPROM, the time required for writing of data is protracted. Therefore, it is still impossible to cover the time required for writing the data with the charging voltage of a capacitor. Thus, the data is written in the EEPROM every second during normal time, too.
However, the writable number of times of an EEPROM is approximately a hundred thousand times in general, and, even when the use time of a portable telephone unit is two minutes a day, the number of write times reaches 87,600 number of times in two years (=60 seconds.times.2 minutes.times.365 days.times.2 years), and it is therefore required to exchange the EEPROM in approximately two years, thus shortening the life of a product.
Furthermore, there is also a method that a RAM is backed up with a primary battery so as to make it nonvolatile, but, in order to back up the RAM for several years with a primary battery only, the capacity of the primary battery has to be made larger.
The present invention has been made for the purpose of solving such problems as described above.